Modern power plants are required to be not only highly efficient but also as flexible as possible in operation. This includes, as well as short start-up times and high load-change speeds, also the possibility of equalizing frequency disturbances in the network. Depending on the requirement profiles of individual networks of different countries and the associated remuneration models, it can therefore be expedient, in particular in the case of combined gas and steam power plants, to make additional power available to the network as quickly as possible during peak load operation via the steam circuit by means of an additional firing. Such gas and steam power plants with additional firing are known for example from DE 10 2010 060 064 A1, EP 1 050 667 A1 and U.S. Pat. No. 3,980,100.
In currently known power plants with additional firing, generally use is made of drum-type boilers. Here, there is a noticeable delay between the additional firing being activated and the quantity of steam produced in the evaporator increasing. The improvement in the steam cooling of affected heating surface pipes, which cooling is directly coupled to the transient behavior of the steam production, is also delayed. In concrete terms, this means that, immediately at the point when the additional firing is switched on, superheater surfaces and reheater surfaces have to at first cope with the increasing heating on the flue gas side with approximately identical steam flow. Conversely, however, this also means that the fluid temperatures—and thus also the wall temperatures—of these heating surfaces can be held within permissible limits only by limiting the power increase of the additional firing. Such a requirement, however, substantially restricts the flexibility of the plant.